Which BMR Formula Should You Use: Mifflin-St Jeor, Katch-McArdle, or Cunningham?

Type your height and weight into most online calculators and you get a single number back, presented like a fact. It's really an estimate from a formula somebody picked, often without saying why, and the three most common candidates (Mifflin-St Jeor, Katch-McArdle, and Cunningham) can disagree by a couple hundred calories a day depending on your body composition. A BMR calculator that picks the right formula for your situation, instead of defaulting to whichever one is easiest to code, gets you closer to a real number. Here's how to think about the choice.

Mifflin-St Jeor: the one with the paper trail

Start here if you don't know your body fat percentage. In 1990, Mifflin, St Jeor, and colleagues measured resting energy expenditure directly, via indirect calorimetry, in 498 healthy adults ranging from lean to obese. They ran the numbers and found something a little humbling for the body-composition crowd: fat-free mass alone explained about 64% of the variation in metabolic rate (R² = 0.64), while plain body weight explained 56% (R² = 0.56). Adding height, age, and sex bumped the full model to 71%. Once you have someone's weight, in other words, knowing their exact lean mass doesn't buy you nearly as much precision as you'd expect.

That's the case for Mifflin-St Jeor as a default: it needs no body fat measurement, and it's been validated more than the alternatives. A 2005 systematic review by Frankenfield and colleagues compared it against Harris-Benedict, Owen, and the WHO equation, and found it landed within 10% of measured RMR more consistently than any of them: 82% of nonobese adults and 70% of obese adults, in the populations that review looked at. The Academy of Nutrition and Dietetics lists it as the preferred equation in its evidence analysis library when body composition data isn't available. It's been checked more thoroughly than either of the alternatives below.

Katch-McArdle: good logic, thinner paperwork

The Katch-McArdle formula, 370 + 21.6 times lean body mass in kilograms, comes from a different premise: forget total weight, calculate off lean mass directly. That's appealing once you know your body fat percentage, because it should, in theory, correct for exactly the blind spot Mifflin-St Jeor has.

Here's the catch. Search for the original validation study behind Katch-McArdle and you won't find one. The formula traces to McArdle, Katch, and Katch's exercise physiology textbook rather than a standalone peer-reviewed paper, and it doesn't turn up in the systematic reviews that stress-test these equations against general and athletic populations. Plenty of fitness sites throw around specific accuracy figures for it, accurate to within some tight percentage, correlated at some precise value with calorimetry, but none of those numbers trace back to an indexed study. Worth knowing before you treat it as gospel.

There's also a striking coincidence. The Katch-McArdle formula is numerically identical, coefficient for coefficient, to the equation Cunningham published in a 1991 peer-reviewed paper: REE = 370 ± 21.6 x fat-free mass. Multiple secondary sources report that the McArdle, Katch, and Katch textbook picked up Cunningham's regression and republished it for a fitness audience under a new name. I couldn't confirm that lineage by reading Cunningham's original paper directly, since it sits behind a paywall, but the coefficients match exactly, which is a hard thing to explain as coincidence. It changes how you should think about the "Katch-McArdle" brand: it may really be Cunningham's formula wearing a gym T-shirt.

Cunningham: built on non-athletes, tested on athletes since

In 1980, Cunningham reanalyzed the classic Harris-Benedict dataset from 1919, 223 subjects in all, and found lean mass was the strongest single predictor of BMR: 500 + 22 times lean mass. His 1991 follow-up refined that using 239 adults, arriving at coefficients now attached to Katch-McArdle's name.

Here's a twist that undercuts a common bit of gym lore. People assume Cunningham's formula was built on bodybuilders, since it's the equation athletes gravitate toward. It wasn't: the 1991 sample excluded 16 subjects who were trained athletes. A 2023 Sports Medicine systematic review and meta-analysis makes this point directly, noting Cunningham's equation was formulated in a population that omitted anyone "athletic." And yet it's become the most-tested equation in athlete research since. That same meta-analysis, which covered 29 studies, found Cunningham used in 21 of them, more than any other equation. Its overall verdict wasn't a coronation of Cunningham, though: a newer equation (Ten-Haaf, from 2014) came out ahead on accuracy and precision in most scenarios, with Cunningham grouped among five equations whose predictions didn't differ significantly from measured values. A 2014 PLOS ONE study on 90 recreational athletes found the Cunningham equation landed within 10% of measured REE for 84.9% of men and 78.4% of women, with bias under 2% in both sexes. Built on non-athletes, it still generalizes well to people who train hard.

Why lean mass matters at all

The physiological reason any of this matters: your organs burn far more per kilogram than muscle or fat does. Wang and colleagues measured this directly in 2010: liver tissue burns roughly 194 kcal per kilogram per day, brain tissue about 233, heart and kidney tissue over 400, against about 13 for skeletal muscle and 4 for fat. Those organs make up a small fraction of body weight but a large share of resting energy expenditure. So two people with identical weight and height can have meaningfully different BMRs if one carries more fat and less lean tissue. A formula built on total weight has to guess at an average body composition. A formula built on lean mass doesn't have to guess, provided you actually know your body fat percentage and that measurement is any good.

Picking one honestly

None of this settles into a tidy hierarchy. There's no clean winner among Mifflin-St Jeor, Katch-McArdle, and Cunningham, because no equation nails everyone. Even Mifflin-St Jeor, the best-validated of the three, misses the 10% accuracy band for something like one in five to one in three people. It helps to hold the whole exercise loosely: two people identical on age, sex, height, and weight can still have resting metabolic rates that differ by as much as 25%, driven by everything a formula can't see, thyroid output, organ size, genetics, how much they fidget. Any equation is drawing a best-fit line through that scatter. If you don't know your body fat percentage, or don't trust the number you have, Mifflin-St Jeor is the sensible default. If you've got a decent body-fat reading (DEXA, a good multi-point caliper measurement), and especially if you train seriously, Cunningham has real peer-reviewed footing and a track record in athletic populations. Katch-McArdle will hand you a similar number for the same underlying reason, with a shakier paper trail behind the name.

MacroMentor's calculator lets you plug in whichever formula fits what you actually know about your body, instead of locking you into one. Try /calculator for a number built around your situation.